Cities eat away at Earth’s best land

By Anil Ananthaswamy in San Francisco

URBANISATION is eating away at the planet’s most fertile and productive land. Although only 3 per cent of land has been built on in the US, the resulting loss in plant growth is enough to offset the gains made by agriculture as shown by a groundbreaking study. If this pattern is repeated in developing countries, the loss of agricultural land could have a serious effect on food supplies.

Forests and cropland also absorb large amounts of the greenhouse gas carbon dioxide. When that land is urbanised, these “carbon sinks” are lost and the carbon is released into the atmosphere. These effects have been hard to quantify because there was no accurate map of urban areas. But now Marc Imhoff of NASA’s Goddard Space Flight Center in Maryland, and his colleagues have shown the drastic effects of urbanisation.

To gather their data, Imhoff and his team used a weather satellite that normally maps moonlit cloud cover at night. But on clear, moonless nights, the satellite picks out city lights, oil flares, forest fires and even lightning strikes. The NASA team used this information to map the mainland US states except Alaska according to three categories of land use&colon; urban, urban periphery and non-urban.

They then calculated how active the vegetation is in each region using data gathered by satellites that map the red and near-infrared frequencies reflected by the chlorophyll of photosynthetic plants. Combining this information with weather conditions such as temperature, humidity and rainfall gave them the amount of plant growth or “net primary productivity” (NPP) in each region.

The researchers found that in the mid-1990s, urban areas cost the

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